【作者】 夏妍；

【导师】 张鉴清； 曹发和；

【作者基本信息】 浙江大学 ， 物理化学， 2014， 博士

【摘要】 钢铁材料由于其优良的性能而被应用于几乎所有的工业领域,成为最重要的结构材料。碳钢是钢铁材料的基础之一,它不仅广泛应用于建筑、桥梁、铁道、车辆、船舶和各种机械制造工业,而且在近代的石油化学工业、海洋开发等方面,也得到大量使用。然而,碳钢在强腐蚀介质、大气、海水、土壤中都不耐腐蚀,绝大多数酸、碱、盐的水溶液对碳钢均有很强的腐蚀性。钢铁的腐蚀造成的经济危害和社会危害巨大,且对钢铁材料的防护费用非常高昂。幸运的是,在碳钢中添加少量合金元素,可以得到价格低廉、耐蚀性能较好的耐候钢。目前,耐候钢的耐大气腐蚀性能已达到普通钢材的2-8倍,被广泛应用于建筑及交通运输领域。作为重要的工业材料,碳钢和耐候钢的腐蚀行为和防护的研究日益受到重视。本论文以碳钢和耐候钢为研究对象,采用微区电化学手段和多种宏观电化学手段,结合物理表征,对薄液膜下的耐候钢、锈层覆盖的钢以及人工钝化膜覆盖碳钢的腐蚀行为进行了详细的研究。本论文的主要研究工作内容和成果包括：采用电化学阻抗谱(Electrochemical Impedance Spectroscopy, EIS),阴极极化曲线,电化学噪声(Electrochemical Noise, EN)等电化学手段,结合扫描电镜/能谱仪(SEM/EDS), X射线衍射(XRD)和拉曼光谱(Raman Spectrum)等考察了不同相对湿度(Relative Humidity, RH)和液膜厚度薄液膜下耐候钢的腐蚀行为。结果表明腐蚀初期液膜厚度为100μm时的腐蚀速率最大,其次是200μm和50μm,400μm液膜下腐蚀速率次之,本体溶液中最小。在100和50gm之间有一个电流密度极大值对应的液膜厚度。50gm液膜厚度下受阳极控制,其余液膜厚度下均受氧的扩散控制。腐蚀中后期,由于液膜厚度的减薄和锈层的生成,腐蚀行为变得复杂：对于50和100gm薄液膜,由于腐蚀初期的快速反应,腐蚀产物在电极表面堆积,腐蚀中后期受阳极控制,腐蚀速率下降；对于200μm薄液膜,在75%和85%RH下的腐蚀行为类似于50和100μm薄液膜,腐蚀中后期,腐蚀速率下降,在97%RH下,腐蚀速率持续上升,但因锈层致密度提高,上升幅度减小；对于400μm薄液膜和本体溶液中,氧的供应不足,锈层疏松多孔,腐蚀速率持续上升,腐蚀后期,液膜减薄到一定程度,氧的供应虽逐渐增多,但锈层逐渐加厚,腐蚀速率上升幅度减小。液膜厚度小于400gm时75%RH下锈层疏松多孔,85%和97%RH下相对较为致密,相对湿度和腐蚀试验时间对锈层的致密性有重要影响。应用微区电化学技术扫描电化学显微镜(Scanning Electrochemistry Microscopy, SECM)和宏观电化学技术如EIS,阴极极化曲线,结合SEM/EDS和XRD物理表征,考察并比较了干湿循环条件中锈层下碳钢和耐候钢的腐蚀行为,发现实验初期,两种钢的腐蚀行为相似,形成的锈层降低了Fe阳极溶解速率,从而提高碳钢和耐候钢的耐蚀性能；后期形成的锈层由于其组成和结构特征的变化,两种钢的腐蚀速率均增加；不同的是耐候钢的腐蚀速率较碳钢大,且自身锈层还原也较碳钢强,有利于锈层的形成,从而有利于长期的防护,但耐候钢的锈层在短期内并没有很好的保护性。两种钢的锈层在于湿循环条件下均不够致密,呈疏松多孔状,其组成主要有晶态的γ-FeOOH, Fe3O4和y-Fe2O3等。在实验周期内,锈层处于非稳定状态,相同干湿循环条件下,耐候钢的锈层较碳钢厚,后期较碳钢锈层致密。采用SECM和宏观电化学手段EIS, Mott-Schottky测试,以及XPS、光学显微镜研究了不同电位下生成的碳钢钝化膜的电化学性质及耐蚀性,发现碳钢在硼酸缓冲溶液中生成的钝化膜对成膜电位有依赖性。随着成膜电位的正移,钝化膜表面电化学反应活性降低。宏观电化学分析表明随着成膜电位的正移,电化学反应电阻增大,点缺陷密度降低,膜的导电性降低。在NaCl溶液中,0.7V电位下的钝化膜溶解较均匀,与其多孔结构有关。0.3V电位下生成的钝化膜的腐蚀速率最小,而0.7V电位下的其次,这是由于后者的多孔结构以及在含Cl-溶液中失去了自修复功能导致。-0.1V电位下的钝化膜腐蚀速率最快,在NaCl溶液中迅速溶解,生成了保护性较强的腐蚀产物。利用COMSOL Multiphysics建立数学模型进行数值模拟,进而求得不同电位下生成的钝化膜与电解液界面的速率常数,并与Origin拟合结果进行比较,发现,两种方法拟合出的数值变化规律相同,但数量级有差别,这可能是因为我们的实验条件受限于Origin拟合公式的设定条件。但COMSOL可以更直观建立腐蚀空间模型,有利于获得更准确动力学参数。更多还原

【Abstract】 As the most important structural material, the steel is applied to almost the whole industrial domain for its excellent properties. Carbon steel is one of the foundation of steel, which is not only used widely in buildings, bridges, railways, vehicles, ships and all kinds of machine-making industrial, but also applied to petrochemical industry and ocean development abundantly. However, the corrosion resistance of carbon steel in highly aggressive medium, atmosphere, seawater and soil is so poor that he economic and social loss that the corrosion of steel brings about is particularly huge, and the cost for protection of steels is extremely high. Fortunately, the weathering steel which is low cost and highly protective can be obtained from the addition of trace alloy elements. At present, the corrosion resistance of weathering steel has reached to2-8times of ordinary steel. As important industrial materials, the investigations of the corrosion behavior and protection of carbon steel and weathering steel are increasingly paid attention to. In this dissertation, the corrosion behavior of weathering steel in TELs, the steels with rust and passive film were investigated via micro-and macro-electrochemical methods combining physical characterization exhaustively. The main contents and results are listed as follows:The corrosion behavior of weathering steel under thin electrolyte layer (TEL) at various relative humidities (RHs) was investigated by cathodic polarization curves, electrochemical impedance spectroscopy (EIS), OM, SEM/EDS, XRD and Raman spectroscopy. The results show that at the initial immersion stage (first2h), the100μm TEL has the largest corrosion rate, following by200μm and50μm, and then400μm. There is a point for the maximum value of corrosion rate between100and50μm, the corrosion under TEL is under the control of O2diffusion except50μm TEL which controlled by anodic process. For the50and100μm TEL, the corrosion rate is relatively large due to the sufficient supplement of oxygen in the initial stage and the rust accumulated, then the anodic reaction predominates the corrosion reaction, which results in the decrease of corrosion rate; In the case ofthe200μm TEL, the evolution of the corrosion resistance is similar to the first type at low RHs (75%and85%RH), while at97%RH, the corrosion rate keeps on increasing with the immersion time, and the relatively compact rust layer forms on the electrode. In the case of400and1000μm TEL, the porous and loose rust layers are formed on the electrode due to the insufficient oxygen in the beginning immersion, and the corrosion rate consistently increases due to the poor protectiveness of rust layer. In the later stage, the extent of the increasing of corrosion rate decrease due to the thick rust layers. For the thinner TELs less than400μm, the rust layers which form at75%RH are always porous and loose, while that of97%RH are compact, indicating that the RH and corrosion period has an significant influence on the density and then protectiveness of rust layer.SECM was used to investigate corrosion behavior of carbon steel and weathering steel under wet-dry cyclic conditions, combined with conventional electrochemical measurements such as polarization curves and EIS, and the physical characterization method of rust layers such as SEM and XRD. The results show that the rust layer formation in the initial stage reduces the Fe anodic dissolution rate, thereby improves the corrosion resistance of carbon steel and weathering steel, while the rust layer formation in the late period of the experimental results in higher corrosion rate due to the changes of its composition and structural characteristics. Weathering steel corrosion rate is higher than that of carbon steel, and the reduction rate of rust is also higher than that of carbon steel, which are conducive to the formation of rust layer, thus contributing to long-term protection of steel, but the rust layer of weathering steel is not very good protection in the short term. Rust layer is loose, porous and not dense enough, and the main composition of rust layer is crystalline γ-FeOOH, Fe3O4and γ-Fe2O3. The rust layer thickness of weathering steel is higher than that of carbon steel in the same wet-dry cycles condition.Passive films formed on carbon steel by different passivation potentials in borate buffer solution and their degradation behavior were studied by EIS, Mott-Schottky analysis and SECM, while the film composition and morphology were investigated by XPS and OM. The results show that the electrochemical reactivity on the surface of passive film is weakened as the film formation potential increases due to slower charge transfer rate. The heterogeneous electron transfer rate constant k calculated from approach curves decreases linearly with substrate potential increasing, resulting from different electrochemical reactivity on the surface of substrate. The electrochemical reaction resistance of the film increases with film formation potential increasing, while the point defect density decreases, indicating the film becomes less conductive. The ratio Fe3+/Fe2+and the percentage of oxyhydroxide species increase simultaneously as the film formation potential shifts from-0.1V to0.7V. Immersed in NaCl solution, the film formed at0.3V has the strongest corrosion resistance followed by that of0.7V which is porous and loose the self-repair ability in NaCl solution. The corrosion rate of-0.1V is fastest and accompanied by the formation of corrosion product with high protectiveness.The probe approach curves for different reaction rate constant of substrate were also simulated by COMSOL Multiphysics, and the heterogeneous rate constants at the interface of passive films formed at different potential and electrolyte are obtained. Comparing the fitting results by COMSOL and Origin, it is can be found that the variation tendency of ks by the potential of the substrate is similar to each other, but there is a difference in the real value of ks. The probable reason is the distinction of our experiment condition and the condition which in the expression of Origin. Since the Comsol simulation is based on the real corrosion spatial model, the fitting result is more accurate than that got from Origin fitting.更多还原